Many treatments exist to improve the gait and function of persons with cerebral palsy (CP). Despite recognizing that muscle weakness is a major impairment in CP, none of the treatments have the aim of increasing muscle strength. The PIs hypothesize that strengthening muscles may be an alternative to procedures that weaken muscles and that ankle strength increases can improve gait and function in persons with CP. A major limitation is that there are presently insufficient data to establish the appropriate number of subjects to conduct a fully powered phase III clinical trial. The purpose of this proposed phase II clinical trial is to establish sample size requirements for a phase III clinical trial to determine if ankle strength increases in the plantarflexors can improve gait and function in persons with CP. Sixteen subjects with spastic diplegia CP will participate in the phase II trial in a 10- to 16-week strength training program designed to strengthen the ankle plantarflexors. Subjects will be assessed for both primary and secondary outcome measures prior to the intervention, and at the end of the intervention. The primary outcome measures will be gait speed and the walk-run-jump dimension of the Gross Motor Function Measure (GMFM-WRJ). The secondary outcome measures will be ankle strength, gait kinematics and kinetics, quality of life, ankle plantarflexor spasticity, dorsiflexion end range of motion, EMG, calf circumference, and pain level. To monitor subject safety during the training sessions, there will be weekly clinical assessments for plantarflexor spasticity, tightness, and pain. To monitor subject strength progress and establish intervention termination, there will be weekly strength assessments. Data from the primary and secondary outcome measures will be used in a repeated measures power analysis to establish sample size for a subsequent phase III clinical trial. Identical data from a historical control group collected in the same manner as the present investigation (R01- NS35830) will be used as a comparison in order to devote all available resources to the intervention group. Accomplishing the phase II trial will position the PIs to conduct a fully-powered multi-center phase III trial. The protocols needed for a multi-center trial will have been established and tested. Substantial preliminary data will be available, and the PIs'experience with recruitment, training regimen, and attrition will indicate the number of sites needed for subject recruitment. The concept of increased strength improving function is well tested in the general population, but not well tested in patients with cerebral palsy. The PIs hypothesize that increased strength in children with cerebral palsy will improve gait, function and quality of life. It will reduce, postpone, or eliminate other procedures that are more debilitating and costly. The present investigation will prepare the PIs to conduct a randomized clinical trial to test their hypotheses.